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We are investigating the self-assembly of partially-filled nanotubes to evaluate the potential of this assembly strategy for solar cell fabrication. Our bottom-up approach relies primarily on density differences between the filled and open ends of the nanotubes to drive assembly of vertically-oriented arrays. Thus far, we have used silica nanotubes partially filled with metal (usually gold) as a test system that provides a large offset in their center of mass; this helps orient the wires as they sediment and facilitates assembly. The assembly mechanism is quite general and therefore should be applicable to a wide range of materials, substrates, and geometries. As columnar structures have shown to be effective in solar cells, we proposed to use this method as a low-cost means to construct nano- and microparticles into functional substrates for solar energy harvesting. Over the past year, we have progressed in our understanding of the assembly mechanism and moved towards systems more compatible with solar applications.

To understand whether this approach can be applied to solar collection arrays, several key questions must be addressed, including: the scalability of the assembly approach, the impact of solution properties (viscosity, ionic strength), and the range of particle types that can be assembled in this manner (size, aspect ratio, center-of-mass, and material composition). This year, we demonstrated assembly on a >10 cm² substrate with standing percentages equivalent to smaller substrates (ca. 60% of particles were standing). This well exceeds nearly all literature self-assembly methods for generating columnar arrays, and requires no microfabrication, vacuum, or applied fields. We also demonstrated high assembly percentages for particles with aspect ratios >40, as well as assembly from more viscous solutions, which resulted only in a slower rate of assembly but no difference in the outcome. These experiments suggest that a wide variety of solvents and/or additivies, e.g., sensitizing dyes, polymers, additional nanoparticles, could be incorporated without preventing assembly; future experiments will test this hypothesis.

We are investigating assembly in microfabricated wells as a means to increase standing percentages, especially for solid nanowires or other particles with centers of mass not well suited for the simple gravity-driven approach described above. This approach also provides control over the spatial location of the standing arrays on a substrate, which will ultimately make it possible to perform post-assembly processing and on-chip integration. In our initial experiments, photoresist microwells increased standing percentages for the partially-filled silica tubes to an average of 95%, with many wells having complete 100% assembly (see figure). While assemblies on open surfaces are disrupted by drying, microwells provided stability during the drying process, making it possible to perform electron microscopy based characterization of the arrays while still in the microwells. We have employed electroplating techniques to anchor the assemblies to the surface, allowing removal of the photoresist microwells without loss of the assembled particle arrays.

Finally, we are in the process of testing the assembly both on planar substrates and in microwells for particles with nonmetallic cores, e.g., Si, Cu₂O, ZnO, CdSe, etc., and new coatings, e.g., TiO₂, ZnO. Microwell patterns will be examined on new substrates including transparent conductive films, e.g., indium-doped tin oxide, carbon nanotube films, and flexible substrates, for example, polydimethylsiloxane (PDMS). Once these particles have been assembled we will begin our characterization studies on both individual and arrays of nanowires. Harvesting solar energy is an important goal in today's society. Through our self-assembly methods, we hope to provide a low-cost, simple route for producing columnar arrays. This approach should be applicable for assembling various desirable combinations of nanomaterials and substrates.